The present invention relates generally to a new family of characteristic antenna structures of reduced size featuring a broadband behavior, a multiband behavior of a combination of both effects. The antennas according to the present invention include at least two radiating structures or arms, said two arms being coupled through a specific region of one or both of the arms called the proximity region or close proximity region.
There exists on the prior-art some examples of antennas formed with more than one radiating structure, said structures being electromagnetically coupled to form a single radiating device. One of the first examples would be the Yagi-Uda antenna (see FIG. 1, Drawing 3). Said antenna consists of an active dipole structure, said active dipole structure being fed through a conventional feeding network typically connected at its mid-point, said dipole being coupled to a series of parasitic dipoles of different lengths, said parasitic dipoles being parallel to the active dipole. The skilled in the art will notice that the present invention is essentially different from the Yagi-Uda antenna for several reasons: first of all, because in the Yagi-Uda antenna the distance between any pair of dipoles is generally constant, that is all dipoles are parallel and no proximity region is included to strengthen the coupling between dipoles. The object of such a coupled parallel dipole arrangement in the Yagi-Uda antenna is to provide an end-fire, directive radiation pattern, while in the present invention the radiating arms are arranged together with the close proximity region to reduce the antenna size yet providing a broadband or multiband behavior.
Another prior-art examples of antennas including two radiating structures coupled together are stacked microstrip patch antennas (“Miniature Wideband Stacked Microstrip Patch Antenna Based on the Sierpinski Fractal Geometry”, by Anguera, Puente, Borja, and Romeu. IEEE Antennas and Propagation Society International Symposium, Salt Lake City, USA, July 2000). In such an arrangement, an active microstrip patch of arbitrary shape placed over a ground-plane is coupled to a passive parasitic patch placed on top of said active patch. It will be noticed that said active and parasitic patches keep a constant distance between them and are not specifically coupled through a specific proximity region on any of the two patches which were closer the adjacent patch. Such a stacked microstrip patch antenna configuration provides a broadband behavior, but it is does not feature a close proximity region as described in the present invention and it does not feature a highly reduced size, since the patches are typically sized to match a half-wavelength inside the dielectric substrate of the patch, while in the present invention the antennas feature a characteristic small size below a quarter wave-length.
A prior art example of monopole and PIFA antennas which are coupled together to feature a broadband behavior are described in “Realization of Dual-Frequency and Wide-Band VSWR Performances Using Normal-Mode Helical and Inverted-F Antennas”, by Nakano, Ikeda, Suzuki, Mimaki, and Yamauchi, IEEE Transactions on Antennas and Propagation, Vol. 46, No 6, June 1998. Again, those examples are clearly different from the antennas described in the present invention because in all of said prior-art arrangements the active elements and the parasitic ones are parallel to each other and do not get the benefit of the close proximity region as disclosed in the present invention, which enhances the broadband behavior while contributing to the antenna miniaturization.
There are some examples of structures in the prior art that include several radiating structures that are not parallel to each other. An example is the V-dipole (see for instance “Antenna Theory, Analysis and Design”, by Constantine Balanis, second edition) wherein there is a minimum distance between the two arms at the vertex of the V-shape, but it should be noticed that such a vertex is the feeding point of the structure and does not form a coupling proximity region between said arms as disclosed in the present invention. In the present invention, the feeding point is specifically excluded from the close proximity region since it does not contribute to a size reduction and/or multiband or broadband behavior as it is intended here. To form a dipole according to the present invention, at least one arm of the dipole needs to be folded such that said folded arm approaches the other arm to form the close proximity region.
Other prior-art examples of antennas with multiple radiating arms are multibranch structures (see for instance “Multiband Properties of a Fractal Tree Antenna Generated by Electrochemical Deposition”, by Puente, Claret, Sagues, Romeu, L{acute over (p)}ez-Salvans, and Pous. IEEE Electronics Letters, vol. 32, No. 5, pp. 2298-2299, December 1996). Again those examples are essentially different to the present invention in which all radiating arms are interconnected through direct ohmic contact to a common conducting structure, while in the present invention at least two of the radiating arms of the antenna must be disconnected and coupled only through said close proximity region.
The skilled in the art will notice that the present invention can be combined with many prior-art antenna configurations to provide new antenna arrangements with enhanced features. In particular, it should be clear that the shape of any of the radiating arms can take many forms provided that at least two arms are included, and said arms include said close proximity region between them. In particular, in several embodiments one or several of the arms according to the present invention take the form of a Multilevel Antenna as described in the Patent Publication No. WO01/22528, a Space-Filling Antenna as described in the Patent Publication No. WO01/54225 or any other complex shape such as meander and zigzag curves. Also, in some embodiments, at least one of the arms approaches an ideal fractal curve by truncating the fractal to a finite number of iterations.